wifi_statistics: add helper to print plain detail

It may be useful to print a station detail in a plain way,
without using any structured output.
Add an helper function for doing so.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
wifi_statistics: add functions to print plain RSSI information

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
wifi_statistics: add "rssi" debugfs file

Add a new "rssi" file in the debugfs directory.
In case of read will behave like stats but will only print
the RSSI detail in a plain form (all the information on one
line).

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
wifi_statistics: add the num_packets attribute with debugfs interface

Signed-off-by: Antonio Quartulli <ordex@autistici.org>